ABSTRACT

The effect of drag reducing agents (DRA) on corrosion and flow regime has been studied in a 10 cm diameter, 18 m long plexiglass flow loop in 50% oil/water mixtures with carbon dioxide gas. Superficial liquid velocities between 0.1 and 1 m/s and gas velocities between 1 and 10 m/s respectively were studied. The corrosion rate was measured for stratified, slug and annular flow. The height of liquid film, slug velocity, and slug frequency were obtained from the video image using a super-VHS camera. The DRA effectiveness was examined for DRA concentrations between 0 and 75 ppm.

Flow regimes maps were determined with 25 and 75 ppm DRA. These results were compared to the flow regime map with no DRA. The results indicate that the transition from stratified to slug flow is obtained at a higher superficial liquid velocities. This resulted in much lower corrosion rates due to the elimination of the highly turbulent slugs.

The corrosion rate for stratified and annular flow did not generally reduce with adding DRA concentrations.

For slug flow, the slug frequency decreased with the addition of 50 ppm DRA. This led to decrease of corrosion rate by almost 50%.

INTRODUCTION

The flow of multiphase mixtures is frequently encountered and internal corrosion of carbon steel pipelines is a common problem in the petroleum industry. It is usually not practical to separate the multiphase mixture at the well site since many oil wells are located in remote sites, e.g. subsea and Alaska Therefore, the multiphase mixture is transported through a large single pipeline to a central gathering station. The distances that the multiphase mixture is transported are often many miles and the pressure drop in these pipelines can be significant. Also, corrosion is a major concern.

DRA can be used to reduce frictional losses in pipeline, which leads to a decrease in the pressure gradient for a given flow rate Lester (1985)?, Nijs (1995)* and Virk and Baher ( 1970)3 have shown that drag reducing agents can be beneficial to reduce pressure drop in turbulent flow, but not in laminar flow Lester (1985)? has indicated that DRA degrades by shear when centrifugal pumps are used. Chang, David and Darby (1983)4 also examined shear degradation effects in 0.46 cm diameter tubes. They have indicated that DRA degrade by age, losing effectiveness after a few days, and shear.

The effect of DRA in single and two phase flow in 2.54 cm diameter horizontal pipes has been studied by Rosehart, Scott and Rhodes (1972)?. They found that drag reduction in two-phase flow is greater than in single-phase flow at the same superficial liquid velocities. These results were restricted to the slug flow regime.

The benefits of DRA added to an existing systems are increased flow rate, reduction of operation costs, and reduction of pressure gradient.

Lee, Sun and Jepson ( 1993)6 have shown flow regime maps and flow patterns for oil-water- carbon dioxide gas in a 10 cm diameter pipeline. They have indicated that the flow regime transitions in oil-water-gas three phase flow differ from those in both gas-liquid and oil-water flow systems. Jepson and Taylor (1989)? have shown that the transitions from stratified to slug flow occurs at higher liquid

This content is only available via PDF.
You can access this article if you purchase or spend a download.